1. Field of the Invention
The present invention relates to a process for electroplating a nonconductive surface, and, more particularly, to a process for electroplating a conductive metal on the inner walls of through-holes in printed-wiring boards.
2. Description of the Background Art
In printed-wiring boards such as double-sided boards and multilayer boards, through-holes are provided in the substrate. The inner walls of the through-holes are electroplated with a conductive metal in order to secure conductivity among the circuits.
As a method of electroplating the nonconductive inner walls of through-holes, Shortt et al. disclose a method of electroplating after providing the through-hole inner walls with electro-conductivity by the application of particles of silver, copper, graphite or the like (U.S. Pat. No. 3,163,588). In this method, however, defects such as pinholes are created in the plated layer of the through-hole inner wall, when excessive plated portions are removed. This inner wall must be re-electroplated, which not only makes the process complicated, but also it cannot be applied at all to the manufacture of reliable high density printed-wiring boards currently required.
In U.S. Pat. No. 3,099,608 Radovsky et al. indicate that a drawback in the case where graphite is used as a conductive layer for electroplating is in the poor deposition of metals to be electroplated.
At the present time, electroless copper plating is applied as the method for plating metal on the through-hole inner walls. However, the electroless copper plating has the following drawbacks: (1) it requires a comparatively long period of time, (2) a number of baths must be monitored all the time during the plating process (required components must be supplied to each bath and sufficient care must be taken so that the baths are not contaminated with components used in the preceding steps, because the baths are extremely sensitive to contamination), (3) a number of washing baths are needed and these baths consume a great quantity of water for washing, and (4) waste water disposal is expensive.
As a method for electroplating without using the electroless copper plating which has these drawbacks, Randolph et al. proposed a method of electroplating after forming a carbon black layer by applying a dispersion of carbon black having an average diameter of less than about 3 .mu.m and a surfactant to the through-hole inner wall, and further forming on this carbon black layer a graphite layer by applying a dispersion of graphite having an average diameter of less than about 1.5 .mu.m and a surfactant (U.S. Pat. No. 5,139,642). This method requires formation of two layers, a carbon black layer and a graphite layer, as base layers for electroplating, resulting in complicated processes and increase in the cost.
As an improvement over U.S. Pat. No. 5,139,642, a process was disclosed in GB 2,274,853. This process was successful in increasing reliability and reducing the cost in electroplating nonconductive surfaces to some extent by forming a layer of graphite using an aqueous dispersion of graphite particles and a binder.
The present inventors have undertaken extensive studies in order to develop a more efficient and reliable method for electroplating a conductive metal on a nonconductive surface, especially on the through-hole inner wall of printed-wiring board, and found that electroplating a conductive metal on a layer of carbon particles, such as graphite particles, produced by application of such particles to the through-hole inner walls and dipping in a strongly acidic aqueous solution provides superb electroplated surfaces without defects such as voids.